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-   -   Forward step (http://www.cfd-online.com/Forums/openfoam-solving/59962-forward-step.html)

 shuo October 22, 2006 21:28

Hi
I made some modifications for the forward step under the solver sonicFoam. I changed the gas to air so that R = 286 J/kg-K, Cv = 723 J/kg-K.
The inlet velocity and temperature were fixed at 500m/s and 300K respectively, giving a Mach number of 1.5. THe pressure was increased to atmospheric conditions 101325Pa. When I tried to run the case the solution blew up due to the ridiculously high Courant number:

Time = 0.002

Mean and max Courant Numbers = 0 83.3333
diagonal: Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
BICCG: Solving for Ux, Initial residual = 1, Final residual = 8.61837e-18, No Iterations 1
BICCG: Solving for Uy, Initial residual = 0.998465, Final residual = 2.95605e-17, No Iterations 1
BICCG: Solving for e, Initial residual = 1, Final residual = 1.4773e-16, No Iterations 1
BICCG: Solving for p, Initial residual = 1, Final residual = 5.63424e-07, No Iterations 9
diagonal: Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
time step continuity errors : sum local = 1.32832e-05, global = 3.05962e-06, cumulative = 3.05962e-06
BICCG: Solving for p, Initial residual = 0.999905, Final residual = 4.4304e-07, No Iterations 36
diagonal: Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
time step continuity errors : sum local = 0.00923335, global = 5.60188e-05, cumulative = 5.90784e-05
ExecutionTime = 0.17 s ClockTime = 0 s

SHuo

 dmoroian October 23, 2006 02:15

I think is normal to diverge i

I think is normal to diverge if you have such a high CFL.
Further on, it is normal to have such a high CFL, if you increase the velocity from 3 to 500. To compute the CFL number you use the velocity not Mach, so even if you preserve Mach but increase velocity, it is normal end up with an increased CFL (in case you preserve also the time step and the mesh).
On short: use a smaller time step or a coarser mesh.

Dragos

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